In the class of hearing aid systems generally referred to as implantable hearing instruments, some or all of various hearing augmentation componentry is positioned subcutaneously on or within a patient's skull, typically at locations proximate the mastoid process. In this regard, implantable hearing instruments may be generally divided into two sub-classes, namely semi-implantable and fully implantable. In a semi-implantable hearing instrument, one or more components such as a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit an audio signal to implanted components such as a transducer. In a fully implantable hearing instrument, typically all of the components, e.g., the microphone, signal processor, and transducer, are located subcutaneously. In either arrangement, an implantable transducer is utilized to stimulate a component of the patient's auditory system (e.g., ossicles and/or the cochlea).
By way of example, one type of implantable transducer includes an electromechanical transducer having a magnetic coil that drives a vibratory actuator. The actuator is positioned to interface with and stimulate the ossicular chain of the patient via physical engagement. (See e.g., U.S. Pat. No. 5,702,342). In this regard, one or more bones of the ossicular chain are made to mechanically vibrate, which causes the ossicular chain to stimulate the cochlea through its natural input, the so-called oval window.
As may be appreciated, hearing instruments that propose utilizing an implanted microphone will require that the microphone be positioned at a location that facilitates the receipt of acoustic signals. For such purposes, an implantable microphone may be positioned (e.g., in a surgical procedure) between a patient's skull and skin, typically at a location rearward and upward of a patient's ear (e.g., in the mastoid region). For a wearer of such a hearing instrument (e.g., middle ear transducer or cochlear implant stimulation systems), undesirable vibration (e.g., non-sound vibration) originating within the user's skull and/or tissue may be detected and amplified by the microphone to an undesirable degree. For instance, a middle ear transducer used with a hearing instrument may create such vibration. In this case, detection and amplification of the vibration can lead to objectionable feedback. Unwanted vibration can also arise naturally from talking or chewing. In both cases, undesired vibrations are transmitted through the user's skull or tissue to the site of the implanted microphone where a component of these undesired vibrations may be received by a microphone diaphragm and where the skin and tissue covering such a microphone diaphragm may undesirably increase the overall vibration sensitivity of the system. In this regard, while proposed implantable hearing aid instruments are sensitive to the sources of undesired vibration, they are intended by design to be sensitive to “ambient” sound vibrations from outside a user's body.
It is therefore desirable to have a means of reducing system response to sources of non-ambient (i.e., undesired) vibration, without affecting the desired ambient sound vibration sensitivity.